This is the 9th in a series of meetings devoted to opportunistic protists that cause serious problems in AIDS patients and other individuals with defective immune systems. The first meeting held in Bristol, England, 1988, had as its sole focus Pneumocystis, a poorly understood organism that was causing pneumonia among immunocompromised patients, especially those with the newly identified Acquired Immune Deficiency Syndrome (AIDS). The meeting was initiated to serve the small, but growing international scientific community interested in working with these unusual, non-cultivatable organisms. At the time, Pneumocystis were relatively unknown organisms and were relatively ignored by the microbiological community. The founders of the meeting, including Dr. Kaneshiro, who remains an active organizer of the Workshops, realized the need for a small meeting where scientists could exchange new techniques and experiences gained from working with these 'difficult' organisms. The basic tenets of these Workshops and those proceeding it were: a free exchange of information facilitated by open admission to the meeting and rapid publication of the proceedings. The 2nd through 8th Workshops grew to include other opportunistic organism-based community group that experienced many of the research challenges facing the Pneumocystis community. The International Workshops on Opportunistic Protists (IWOP) now include research presentations on Cryptosporidium; the microsporidia; Toxoplasma gondii; free-living amebae turned pathogenic, and other immunodeficiency-dependent eukaryotic microbial pathogens. Today, IWOP is regarded as the most important meeting for most researchers of Pneumocystis, Cryptosporidium, and the microsporidia. IWOP-9 is scheduled for June 20-24, 2006 in Lisbon Portugal, a venue in Southern Europe that is expected to attract a large number of participants especially from the continent where many investigators in this field work. As in several IWOP meetings in the past, the 2006 Workshops will be held jointly with the annual meetings of the International Society of Protistologists ( formerly Society of Protozoologists).